Pathogenesis of bacterial meningitis (a new R21) Abstract Bacterial meningitis continues to be an important cause of mortality and morbidity, and a major contributing factor to such mortality and morbidity is our incomplete knowledge on the pathogenesis of this disease. Escherichia coli (E. coli) and group B Streptococcus (GBS) are the two most common bacteria causing neonatal meningitis, but the pathogenesis of E. coli and GBS meningitis remains incompletely understood. Several lines of evidence from human cases and experimental animal models of E. coli and GBS meningitis indicate that E. coli and GBS penetration into the brain occurs initially in the cerebral microvessels. Since E. coli and GBS penetration into the brain occurred in the cerebral microvasculature, we developed the blood- brain barrier model with human brain microvascular endothelial cells (HBMEC) to investigate E. coli and GBS penetration of the blood-brain barrier. Our HBMEC monolayer, upon cultivation on collagen-coated Transwells, exhibits spatial organization of tight and adherens junction proteins as well as a polarized monolayer, a unique property of the blood-brain barrier endothelial cells. Meningitis isolates of E. coli and GBS have been shown to exhibit the ability to invade the HBMEC monolayer and the ability of HBMEC invasion is shown to be correlated with penetration into the brain. We, therefore, used E. coli and GBS invasion of HBMEC monolayer as a biologically relevant approach for discovery of novel targets affecting E. coli and GBS penetration of the blood- brain barrier, the essential step in the development of meningitis. A feasibility of our approach is shown by our identification of epidermal growth factor receptor (EGFR) tyrosine kinase affecting E. coli and GBS penetration of the blood-brain barrier. Of particular relevance to bacterial meningitis is our preliminary demonstration that Streptoccous pneumoniae (S. pneumoniae), the leading cause of bacterial meningitis in children and adults, also exploits EGFR for penetration of the blood-brain barrier. EGFR has not been recognized for its contribution to E. coli, GBS and S. pneumoniae meningitis. Elucidation of EGFR for its contribution to E. coli, GBS and S. pneumoniae penetration of the blood-brain barrier will, therefore, enhance our knowledge on the pathogenesis of bacterial meningitis. The innovative aspect of this exploratory application is to investigate a new target exploited by common meningitis-causing bacteria (E. coli, GBS and S. pneumoniae) for their penetration of the blood-brain barrier. The information derived from this application will provide a new paradigm for investigating the pathogenesis, prevention and therapy of bacterial meningitis.